Without limiting the scope of the invention, its background is described in connection with MOS technologies, as an example.
As MOS technology advances, increasingly thinner gate oxide layers are required. With thinner layers comes an increased demand for improved efficiency, decreased failure rates, and greater control over the process. One problem that is widely recognized in the wafer manufacturing process is gate oxide charging. Several steps in the process have been implicated in the charging process, including: ion implantation, ashing, surface scrubbing, and etching.
Gate oxide damage may also occur during the subsequent deposition of an oxide layer. For example, damage due to a plasma enhanced TEOS or silicate deposition, leading to degradation in hot carrier lifetime, has been reported to vary with the thickness of the oxide layer deposited. Gate oxide damage has been shown to be reduced by a protective layer of polysilicon with a thickness of 150 nm. The 150 nm polysilicon layer was reported to block charging effects during subsequent processing.
The conventional theory of plasma-charging damage, on the other hand, relies strongly on the antenna's ability to collect charges from the plasma. In contrast, a process known as "photoconduction" has been proposed as a mechanism for charging damage that increases as the thickness of the deposition film increases.
Both theories, however, fail to explain the continuously increasing damage as more dielectric is deposited. The failure of the photoconduction theory is particularly true since even a thin layer of dielectric can prevent the antennae from further charging.
Alternatively, others have proposed that the correlation between charging and oxide deposition indicates that the thickness of the layer causes a "photocurrent" to fall off quickly as the inverse square of the distance that the photons have to travel. In the vertical direction, the fall off would be even faster because less photons would penetrate the thickness of the oxide.
Therefore, the current understanding in the field is that thickness dependent damage and the effect of saturation is a combination of the photocurrent fall off and damage saturation, whether the photocurrent does or does not fall off. Furthermore, the photocurrent fall off would also make the photoconductive effect a very local phenomenon.